Redox-zones in the biomass
7.4.
If the specific volumetric area in the filter, ro, is 100 m 2 /m 3 , the volume of the filter, V2, is
V2 = A2·/ro = 4,900 m 2 /1 oo m- 1 = 49 m 3
(corresponding to a hydraulic retention time for an empty filter of approx. 40 min.).
Well, that was a long example. What about its credibility? We will answer that question by
quoting the German philosopher Georg Wilhelm Friedrich Hegel: "If reality does not
want to abide by the idea- it's too bad for reality".
Redox-zones in the biomass
In treatment plants where biomass is present in the form of biofilms or floes, the
removal may be limited by diffusion. This will usually be dominant in biofilters, but
it may also play a role in activated sludge plants. In both cases processes may occur
which cannot immediately be explained without understanding the possible limitation of the diffusion in respect of the reaction. It is important for the understanding
of the observed phenomena that the diffusion does not just lead to limitations in the
reaction rate; it is equally important to understand that the diffusional limitation
may lead to layers of redox- zones in the biomass. The section on simultaneous
aerobic removal of organic matter and denitrification in the rear of the biofilm is a
good example; but it is just an example of something of a more general nature.
Fig 7.27 is an example of layers of redox-zones as they may occur in a biofilm. In
each layer is indicated whether the substance is removed or produced. In zones
where the substance is removed, the curvature is negative. In zones with production
the curvature is positive, like the second derivative of the concentration profile
(Section 5.1).
The outer layer, exposed to the bulk, oxygen-rich water, is aerobic due to the
diffusion of oxygen into the biofilm. It is the penetration depth of the oxygen which
limits the aerobic removal in the reactor. In this zone the substances from the
wastewater, which can be degraded aerobically, are oxidized (especially diffusible
organic matter and ammonium), as described in Chapters 5 and 6. It should be
realised that it is not just the substances coming from the outside which may be
oxidized. A diffusion of reduced substances from the anaerobic conversion of
organic matter in the internal layers may diffuse to the outer layers of the biofilm to
be oxidized there. In the figure, this is illustrated with the profile for ammonia.
If nitrate is present in the water and/ or if nitrate is produced by nitrification in the
aerobic zone, nitrate will diffuse into the oxygen-free zone where denitrification will
occur (the anoxic zone). The denitrification requires the presence of a reductant
which has so far been described solely as originating from the diffusion (into) from
the water. This reductant may also originate from the internal, reduced zones in the
biofilm. This may cause phenomena which will influence the net removal.
268
7.4.
If the specific volumetric area in the filter, ro, is 100 m 2 /m 3 , the volume of the filter, V2, is
V2 = A2·/ro = 4,900 m 2 /1 oo m- 1 = 49 m 3
(corresponding to a hydraulic retention time for an empty filter of approx. 40 min.).
Well, that was a long example. What about its credibility? We will answer that question by
quoting the German philosopher Georg Wilhelm Friedrich Hegel: "If reality does not
want to abide by the idea- it's too bad for reality".
Redox-zones in the biomass
In treatment plants where biomass is present in the form of biofilms or floes, the
removal may be limited by diffusion. This will usually be dominant in biofilters, but
it may also play a role in activated sludge plants. In both cases processes may occur
which cannot immediately be explained without understanding the possible limitation of the diffusion in respect of the reaction. It is important for the understanding
of the observed phenomena that the diffusion does not just lead to limitations in the
reaction rate; it is equally important to understand that the diffusional limitation
may lead to layers of redox- zones in the biomass. The section on simultaneous
aerobic removal of organic matter and denitrification in the rear of the biofilm is a
good example; but it is just an example of something of a more general nature.
Fig 7.27 is an example of layers of redox-zones as they may occur in a biofilm. In
each layer is indicated whether the substance is removed or produced. In zones
where the substance is removed, the curvature is negative. In zones with production
the curvature is positive, like the second derivative of the concentration profile
(Section 5.1).
The outer layer, exposed to the bulk, oxygen-rich water, is aerobic due to the
diffusion of oxygen into the biofilm. It is the penetration depth of the oxygen which
limits the aerobic removal in the reactor. In this zone the substances from the
wastewater, which can be degraded aerobically, are oxidized (especially diffusible
organic matter and ammonium), as described in Chapters 5 and 6. It should be
realised that it is not just the substances coming from the outside which may be
oxidized. A diffusion of reduced substances from the anaerobic conversion of
organic matter in the internal layers may diffuse to the outer layers of the biofilm to
be oxidized there. In the figure, this is illustrated with the profile for ammonia.
If nitrate is present in the water and/ or if nitrate is produced by nitrification in the
aerobic zone, nitrate will diffuse into the oxygen-free zone where denitrification will
occur (the anoxic zone). The denitrification requires the presence of a reductant
which has so far been described solely as originating from the diffusion (into) from
the water. This reductant may also originate from the internal, reduced zones in the
biofilm. This may cause phenomena which will influence the net removal.
268